time usage and temperature of course it degrades the degradation rate is just likely so slow that one doesnt need to worry about it until the warranty period is long up if youre planning on running the car into the ground sure change the tranny oil at like k miles unless youre taking it to the drag strip or constantly slipping the torque converter in which case parts will wear faster anyway new fluid or not its probably fine even past that though friend of mine changed his tranny oil at miles and based on the oil quality said it was basically a waste of time